The production of bleached chemical pulp is a major industry around the world. More than 50 million tons of bleached pulp is produced annually. Bleached chemical pulp is the largest component of all types of white paper, including that used in photocopy paper, writing paper, and paper packaging. In addition, bleached chemical pulp is also used to impart strength to less expensive grades of paper, such as newsprint. Bleached chemical pulp has large markets because of its high degree of whiteness and cleanliness, the stability of the whiteness, its high strength, and the ease and uniformity of the printing surface it provides. These attributes are obtained when lignin, which is colored and decreases the interfiber bonding of the cellulose, is almost completely removed from the pulp.
In the process of chemical pulping, the furnish (or feedstock) primarily consists of wood chips which are added to a reaction chamber, known as a digester, and are treated with chemicals to dissolve lignin in the pulp. There are several chemical pulping processes known in the art. Two of the major chemical pulping processes are kraft pulping, in which the pulp is cooked in alkaline liquor, and sulfite pulping, in which the pulp is cooked in acidic liquor. Both kraft pulping and sulfite pulping may be performed in batch or continuous digestors.
One of the main purposes of the pulping process is to release lignin which binds cellulose fibers in the feedstock. Pulping dissolves 85% to 95% of the lignin in the feedstock material. Following the pulping stage, the pulp is washed with water to remove dissolved lignin.
While pulping removes most of the lignin in the feedstock material, it is not capable of removing all the lignin without destroying the cellulose fibers of the feedstock. The remaining lignin is removed from the pulp by bleaching.
A pulp bleaching process may consist of many stages. For example, following pulping, a pulp bleaching process may comprise an alkaline oxygen delignification stage (O), an enzymatic treatment stage (X), one or more chlorine dioxide stages (D), and one or more alkaline extraction stages (E). A pulp bleaching process may also comprise one or more water washes or alternatively, each stage may comprise a water wash as a final step of the stage. Thus, a representative pulp bleaching sequence in which pulp is bleached using three chlorine dioxide stages and two alkaline extraction stages may be represented as D-E-D-E-D. Similarly, a pulp bleaching sequence wherein pulp is subjected to an alkaline oxygen delignification stage, an enzymatic treatment stage, three chlorine dioxide bleaching stages and two alkaline extraction stages wherein each stage is followed by a water wash may be represented by O-X-D-E-D-E-D.
It is common for mills to perform an alkali-oxygen delignification stage prior to carrying out chemical bleaching of pulp. This process consists of reacting the pulp with oxygen and alkali at high temperatures (approximately 100° C.) for a period of about one hour. Alkali-oxygen delignification reduces the amount of lignin in the pulp by 35-50%, but this process is harsh on the pulp and is often accompanied by destruction of some of the cellulose fibers in the pulp. Following alkali-oxygen delignification, the pulp is washed as described earlier to remove solubilized lignin.
The next bleaching stage after alkali-oxygen delignification is usually chemical bleaching with oxidative chemicals, the most prominent being chlorine dioxide (ClO2). However, several processes have been described which may facilitate or enhance bleaching of pulp prior to chemical bleaching. For example, an enzymatic treatment stage with xylanase may be used to enhance the bleaching of pulp prior to chemical bleaching.
Xylanases are used in the pulp and paper industry to enhance the bleaching of pulp and to decrease the amount of chlorinated chemicals used in bleaching stages (Erickson, 1990; Paice et al., 1988; Pommier et al., 1989). There have been several mechanisms proposed for the bleaching action of xylanase. One is that lignin is connected to crystalline cellulose through xylan and xylanase enzymes facilitate bleaching of pulp by hydrolysing xylan, releasing coloured lignin from the pulp. A second proposed mechanism is that xylanase removes xylan thereby improving the alkali extractability of the pulp. Regardless of the mechanism, xylanase treatment allows subsequent bleaching chemicals such as chlorine, chlorine dioxide, hydrogen peroxide, or combinations of these chemicals to bleach pulp more efficiently than in the absence of xylanase. Pretreatment of pulp with xylanase prior to chemical bleaching increases the whiteness and quality of the final paper product and reduces the amount of chlorine-based chemicals which must be used to bleach the pulp. This in turn decreases the chlorinated effluent produced by such processes.
Xylanases have been isolated from a variety of organisms including bacteria and fungi. Generally, fungal xylanases exhibit optimal activity at acidic pHs, in the range of about 3.5 to 5.5, and a temperature of about 50° C. In contrast, bacterial xylanases exhibit optimal activity at pH 5 to pH 7 and a temperature optimum between 50° C. and 70° C. However, there are other xylanase enzymes which exhibit optimal activity under other conditions. For example, U.S. Pat. No. 5,405,789 to Campbell et al., discloses construction of thermostable mutants of low molecular mass xylanase from Bacillus circulans. U.S. Pat. No. 5,759,840 to Sung et al., discloses modification of a family 11 xylanase from Trichoderma reesei to improve thermophilicity, alkalophilicity and thermostability as compared to the natural xylanase. U.S. Pat. No. 5,916,795 to Fukunaga et al., discloses a thermostable xylanase from Bacillus. A publication entitled “Xylanase Treatment of Oxygen-Bleached Hardwood Kraft Pulp at High Temperature and Alkaline pH Levels Gives Substantial Savings in Bleaching Chemicals” to Shah et al., (J. of Pulp and Paper Science, vol 26 No. 1 January 2000, which is herein incorporated by reference) discloses treating oxygen delignified hardwood pulp with xylanase from Thermotoga maritima at pH 10 and 90° C. and subsequently bleaching the pulp. These documents disclose using xylanases to enzymatically treat pulp prior to chemical bleaching. However, none of these documents suggest using xylanases to treat pulp after a chemical bleaching stage.
The next stage in a typical pulp bleaching process is usually chlorine dioxide bleaching with chlorine dioxide, chlorine or in some instances, a combination of chlorine dioxide and other oxidative bleaching agents. For example, the first chlorine dioxide stage in a chemical bleaching process is often called the Do or D100 stage. Subsequent chlorine dioxide bleaching stages are referred to as D1, D2 and so on. For mills that bleach pulp without an alkali-oxygen delignification stage, the Do stage is the first chemical bleaching stage. The Do stage is usually carried out at pH 1.5 to 3.0. In a small but decreasing number of mills, up to 30% to 50% chlorine gas may be added to ClO2 in an effort to achieve a higher efficiency of lignin removal. Such a stage is referred to as a CD stage. After a Do or CD stage, the pulp is washed with water, and alkaline extracted. Alkaline extraction is carried out by adjusting the pH of the pulp to 9.0 to 12.0 with sodium hydroxide or sodium carbonate at a temperature between 60° C. to 120° C. for a period of 30 to 90 minutes. After the alkaline extractions stage, the pulp is washed with water. The sequence of chlorine dioxide bleaching stage, wash and alkaline extraction is repeated until the pulp is suitably bleached. In most cases, two to three rounds of acidic and alkaline bleaching, alternating between chlorine dioxide stages and alkaline extraction stages, are required before the pulp is suitably bleached.
In all present commercial applications, xylanase use comprises a xylanase treatment stage prior to the first chlorine dioxide stage. This results in a pulp with increased brightness compared to pulp treated in a similar manner but without xylanase treatment. Alternatively, a specific brightness level can be achieved using a smaller amount of bleaching chemicals when the pulp is treated with xylanase prior to bleaching, compared to pulp that is not treated with xylanase before bleaching.
U.S. Pat. No. 5,645,686 discloses a process for bleaching a chemical paper pulp by means of a sequence of treatment stages involving at least one stage with hydrogen peroxide and at least one stage with a peroxyacid. The patent also discloses a xylanase treatment stage in combination with the pulp bleaching sequence. The patent does not suggest subjecting pulp to a xylanase treatment stage after a chlorine dioxide stage in a pulp bleaching process which employs only chlorine dioxide bleaching stages. Further, there is no teaching as to whether a xylanase treatment stage after a first chlorine dioxide bleaching stage may be more effective in enhancing the bleaching of pulp compared to a pulp bleaching sequence wherein xylanase treatment is performed prior to the first chlorine dioxide bleaching stage.
WO 91/05908 discloses a process for producing bleached lignocellulosic pulp having reduced organically bound chlorine and reduced brightness reversion. The process entails treating pulp with xylanase after a chemical bleaching stage which primarily employs chlorine. The reference teaches that xylanase treatment after a chlorine bleaching stage is not as effective at bleaching pulp as xylanase treatment prior to a chlorine bleaching stage. The reference does not address whether a chlorine dioxide bleaching stage, as employed now by most mills, followed by a xylanase treatment stage may be capable of enhancing the bleaching of pulp.
A publication entitled Xylanase Pre- and Post-treatments of Bleached Pulps Decrease Absorption Coefficient by Wong et al., (2000. J. of Pulp and Paper Science Vol 26 No. 10 377-383, which is herein incorporated by reference) teaches xylanase treatment of pulp as a final stage of a partial or complete chemical bleaching process. However, the reference teaches that xylanase treatment of pulp after chemical bleaching increases the brightness of pulp by a smaller amount than does conventional xylanase treatment of pulp before chemical bleaching.
While the xylanase treatments in pulp bleaching processes generally result in enhanced pulp bleaching compared to equivalent pulp bleaching processes which do not comprise xylanase treatment, there is a need in the art to increase the efficiency of the xylanase treatment. The pulp industry is under pressure to decrease the use of chlorine-containing bleaching chemicals, such as chlorine and chlorine dioxide, and thus, any method or process which can be integrated into a pulp bleaching process to reduce the use of chlorine-containing bleaching chemicals or the toxic effluents produced by the use of such chemicals would be an important and valuable asset to the pulp industry. The industry would also save money by using less chemicals such as, chlorine dioxide in bleaching stages, and sodium hydroxide and hydrogen peroxide in alkaline extraction stages. Improving the efficiency of xylanase treatment would address these concerns by further decreasing chemical usage.
There is a need in the art for novel methods and more efficient methods of bleaching pulp. Further, there is a need in the art for methods, or processes which can be integrated into existing pulp bleaching processes to increase the efficiency of the bleaching process and reduce the use of chlorine containing bleaching compounds or the toxic effluents produced by the use of such chemicals. There is also a need to save money by decreasing chemical usage.
It is an object of the invention to overcome drawbacks in the prior art.
The above object is met by a combination of the features of the main claims. The sub claims disclose further advantageous embodiments of the invention.